This invention relates to testing techniques for mixtures and composite materials, and, more specifically, to a process for measuring nondestructively the fractions of the phases of working specimens.
Many of the materials used in modern technology, particularly those used for their structural properties, are mixtures of several phases which generally retain their inherent character within the mixture. One important class of such mixtures is composite materials, wherein at least two distinct phases are bonded together to from a single material. In a typical structural composite material used in aerospace applications, oriented, high-strength, low ductility graphite, carbon, Kevlar or glass reinforcement fibers are embedded in a resin matrix which binds and protects the fibers. The properties of the resulting composite material reflect the high strength and elastic properties of the reinforcement fibers, yet the composite material is formable and usable in a variety of applications.
One of the most important parameters characterizing such a composite material is the weight (or, equivalently, the volume) fractions of the phases. That is, such a composite material can be described as containing a particular weight fraction of a first phase, another particular weight fraction of a second phase, and so forth, so that the weight fractions of all the phases total 1.0. The greater the amount of a particular phase present in the composite material, the greater is its influence on the overall or total composite material properties.
For many properties such as elastic modulus, the total composite property is the linear sum of the same property for each phase times the volume fraction of that phase present, summed over all of the phases making up the composite material. This relationship is called the rule of mixtures, and is obeyed exactly for some properties and nearly exactly for many other properties. In any event, the functional relationship between the properties of individual phases and the total composite property is important in systematic design work using composite materials. One of the attractive features of composite materials is that they may be tailored to exhibit specific required properties by varying the fractions and arrangement of the phases. The relationship between the properties of individual phases and the total composite property has been the focus of much scientific and engineering attention.
Once a composite material has been designed to have a particular combination of properties, it must be manufactured to the design specifications and inspected to be certain that the manufacturing process actually resulted in the desired material. After manufacture and during service, the composite material must be inspected periodically to insure that its properties have not changed during use. For example, absorption of moisture by the nonmetallic matrix, due to environmental exposure, can seriously degrade the composite properties. In both types of inspection procedures, measurement of the weight fractions of the phases is necessary because the properties of the composite material depend directly upon the weight or volume fractions of the phases, in the manner previously discussed.
The measurement of the weight fractions of the phases in the final composite material is not easy to perform, because portions of the phases are buried inside the composite material and are not readily visible to the naked eye nor measurable by external instruments. The most common approach to the measurement of the fractions of the phases during manufacturing is to section random samples of the material so that the internal structure can be inspected and the volume fraction determined (which then can be converted to a weight fraction, if desired), or to remove the matrix phase and weigh the amount of the fiber reinforcement phase to calculate a weight fraction (which then can be converted to a volume fraction, if desired, with knowledge of the densities of all of the phases of the composite material and the density of the composite material itself). In either event, the specimen that is investigated is destroyed and cannot be reused. A destructive testing program of this type usually requires a cost expenditure of about $40 to $150 per specimen examined, which cost tends to reduce the number of specimens tested and the reliability of the testing program. The testing procedure requires about 1/2 to about 3 hours, preventing real time control of the manufacturing process based upon the measurements.
Service determinations of weight or volume fraction are even more difficult, since the composite material is usually boded into a structure which cannot be sectioned or dissolved. The composite material will also have been subjected to various changes during its service lifetime, which may influence its proprieties. One cannot therefor assume that the composite material in service has phase fractions and phase properties within acceptable limits, simply because the original material was acceptable. As an example, many resin matrix materials absorb moisture during service, changing the effective phase weight fractions of the matrix and the fibers, with respect to the weight of the composite, and the physical properties of the matrix. Because of this possibility, it is necessary to determine the phase fractions and sometimes the properties during service, to be certain that the composite material properties remain within the design limits.
Various types of measurement techniques have been developed to gain information about the internal structure of mixtures and composite materials, including the destructive techniques described above. However, all suffer from the shortcoming that quick, accurate, and inexpensive measurements of the phase fractions of working specimens cannot be made in a nondestructive fashion. Accordingly, there exits a need for a new technique for measuring the weight or volume fractions of the phases of a mixture, such as composite materials and the many other types of mixtures whose structures must be understood and characterized. The present invention fulfills this need, and further provides related advantages.